In order to handle sail and rig on a sailing boat, lines of different types are usually used. Examples of functions that are guided by means of lines are:                Hoisting sail        Sheeting of sail        Taking in sail        Stretching sail, e.g., outhaul and downhaul        Stretching running rig, e.g., yang block and backstays.        
Common denominator for all types of lines is that they stretch more or less during load. In many applications, this is a disadvantage. As an example, hoisting sail may be mentioned:
A sail is hoisted by means of a line (halyard), which runs through a sheave in the upper part of the mast. Persons hoisting the sail are on deck level. The sail is hoisted until it is in top or until it has reached a certain predetermined level, e.g., upon taking in. Normally, a winch is required in order to get sufficient stretch of the halyard and thereby also of the leading edge of the sail. When the sail has been hoisted to the desired height, the halyard is locked on deck level, usually in a so-called rope-clutch. During sailing the halyard will be exposed to additional load because of the influence of the wind on the sail. This extra load entails that the halyard stretches, which in turn leads to the sail sliding down. This may in turn lead to undesired folds in the sail and thereby impaired aerodynamics, which results in a deterioration of the sailing properties of the boat.
Traditional sail canvas stretches relatively much in comparison with more modern racing cloths, which entails that when the leading edge is tensioned, it stretches approximately as a very stiff rubber elastic. When then the sail slides down due to the halyard stretching, the sail contracts somewhat, and no folds arise due to the remaining tension of the leading edge.
In the last few years, the development is toward more and more unelastic sail canvas—this in order to get sails with more stable shape. When an unelastic sail slides down due to the halyard stretching, folds are formed along the leading edge of the sail when the tension of the leading edge disappears. In order to compensate for this, it is necessary to afterstretch the halyard. The alternative is to haul-in in the downhaul (Cunningham). However, upon additional wind increase, the halyard will be exposed to additional load, which entails that the halyard stretches additionally, which in turn results in the sail sliding down additionally, which in turn results in new folds in the sail and so on.
In order to reduce the problems of unelastic sails, more unelastic lines have been developed. A traditional polyester line stretches approx. 3-4% (upon work load) while a modern line manufactured from e.g., SPECTRA™ or VECTRAN™, shows strain values of down to 1%. However, these lines are very expensive and do not solve the problem, since a strain of 1% yet corresponds to 150 mm strain if the line is 15 meters.